Frequency modulation system



July 21, 1942. E. H. ARMSTRONG FREQUENCY MODULATION SYSTEM Filed Jan. l2, 1940 mvENToR {JW/)7 #Arms/rang w ww E N R O T T A Patented July 21, 1942 FREQUENCY MoDULA'rIoN sYS'rlEMV Edwin H. Armstrong, New York, N. Y. Application January 12, 1940, Serial-N0. l313,497

6 Claims.

This invention has for its object the elimination of disturbances-resulting from frequency changes in radio transmitting systems, particularly of the type described in my U. S. Patent #1,941,068 for producing Wide band frequency modulated signals. In this type of transmitter the frequency modulated signal is produced. bychanging the phase of a current derived from a part of this specification, the single figure illustrates the general arrangement of theapparatus which will be understood from the following desciiption. An arrangement now in common use for producing a frequency modulated current of 100 k. c. deviation at approximately 40 megacycles consists of a crystal controlled oscillator having a frequency of 200 k. c., a phase shifting Vdevice therefor, and a series of frequency multipliers. The frequency multiplication is usually accomplished in two steps, the first stage being up to some convenient multiple of the 200 k. c. from which it is heterodyned down to some relatively lower frequency that is a sub-multiple of the frequency to be radiated, and this in turn is multiplied up to the final Value. It has been common practice to multiply up to a frequency of 12.8 megacycles, which is then heterodyned down to some frequency in the vincinity of 900 k. c. This frequency is then in turn multiplied up to the vicinity of 40 megacycles. In a specific case for producing a 42` megacycle wave a heterodyning current of l11.925 megacycles would be combined to produce a difference frequency of 875 k. c. which would them be multiplied 48 times to produce a 42000 kilocycle current. The total frequency multiplication amounts to 3072 times.

It has been found that the frequency of the initial crystal oscillator is not a constant quantity, but varies minutely due to irregularities of filament emissions and to various other effects, and while these changes are extremely minute, after a multiplication of 3000 fold they become perceptible. The present invention shows how Cil (ci. 17e-171.5)

their original value that they are of negligible importance. y

Referringv now to the single figure of thedrawing, the initial oscillator is represented by I,

fwhich is crystal controlled, 2, 3`represent the usual phase shifting arrangement, and 4 the correction amplifier for making the phase shift of 2, 3 inversely proportional to the frequency of the modulating current. firepresents an amplifier for the phase shiftedcurrent output of 2, 3, and 6-II represent a series of frequency doublers which produce currents of the frequency indicated on the diagram. The 12,800 kilocycle current output of the doublers lvl is fed into a converter I9, and this converter, in the example which is selected to give a final frequency of 42000 k. c., is supplied With a heterodyning current of 11,925 k. c. The resultant output current is 875 k. c. This current is amplified by an amplifier 23, and multiplied by a series of doublers 24-28 to 14000 k. c. tripled by 29 to 42000 k. c., and then amplified by the power amplifier 30 into the antenna 3l.

The arrangement so far described is in accordance with standard practice with this exception. In the present practice the heterodyning current would be produced by a second crystal oscillator. Any changes in the frequency of the initial oscillator'would produce a change in the diiierence frequency of the beat with the frequency of the second oscillator, the numerical change being increased some 64 fold due to the multiplication of the doublers E-I I. The present invention proposes to eliminate this change by deriving the heterodyning frequency from the initial oscillator and subjecting it to the same number of multiplications as the current supplied to the converter I9 through th main path has had. This is accomplished in e following way. A part of the output of the oscillator I at 200 k. c. is used to excite an amplifier I2, and the output of this amplifier is then doubled through the system Iii-I8 to 12,800 k. c. through substantially the same type of system as is used in the main path S-I I.4 The output of -the last doubler I8 is fed into a converter 20 Where it is combined with a current having a frequency which is a sub-multiple of the frequency which it is desired to radiate and suiiiciently low in value to result in the desired number of multiplications. In the present case a frequency of 875 k. c. is selected for the second oscillator 2|. The combination of this frequency with the 12,800 k. c. .frequency producesq of course, the usual sum and difference to reduce them to such a small percentage of frequencies of 13,675 and 11,925 k. c. Either frequency may then be selected through a lter 22. In the present case the difference frequency of 11,925 k. c. is used and this, in combination with the frequency of 12,800 k. c. from the doubler I I gives an output frequency tor the converter I9 of 875 k. c. It can be readily seen that any change of the frequency of the oscillator I, at Whatever rate, takes place in the outputs of the doublers I I and I8 equally, so that they are neutralized in the converter I9, and only the changes in frequency, or phase, introduced in the main path by the arrangement 2, 3 are passed on to the antenna. The frequency of the radiated wave is thus controlled by the oscillator 2|, and since the number of multiplications in the frequency from this point on to the final frequency is only lill, the minute changes here are of no importance.

It will, of course, be obvious that various other arrangements and means may be employed for minimizing the disturbances created in the oscillating system. The essential feature of the iiivention is to provide means whereby they cancel themselves o ut.

I have described what I believe to be the best embodiments of my invention. I do not Wish, however, to be confined. to the embodiments shown, but what I desire to cover by Letters Patent is set forth in the appended claims.

I claim:

1. The process of eliminating variations in frequency due to variations in the initial source of oscillations of a frequency multiplying system which consists in providing a pair of paths each containing equivalent multiplying elements, introducing an additional frequency into one of the paths, beating the resulting frequency of the one path with that of the other to produce a third frequency from which said variations are substantially eliminated, and providing modulating means for affecting the frequency in the one path unequally from the effect thereof on the other, whereby the said modulations are caused to appear in the third frequency, and transmitting a signal derived from the third frequency.

2. The method of eliminating variations in frequency due to variations in the initial source of oscillations of a frequency multiplying system which consists in providing a pair of paths containing similar multiplying elements, introducing an additional frequency into one of the paths, modulating over a range the frequencies of the currents in only one of the paths, beating the resulting frequency of the one path with that of the other to produce a third frequency in which th said modulations appear but from which the variation of the source are substantially eliminated, multiplying the third frequency and transmitting signal currents of the resulting multiplied frequency.

3. The method of eliminating variations in frequency due to variations in the initial source of a frequency multiplying system which consists in providing a pair of paths, multiplying the frequency of the source in each of said paths by substantially the same amount, introducing into one of said paths a frequency which is a submultiple of the frequency to be transmitted, beating the resulting frequency in said one path with the frequency in the other path to produce a third frequency from which said variations are quency in said other path whereby the modulations are caused to appear in the third frequency, multiplying the thirdfrequency so as to produce the frequency to be transmitted and transmitting currents of the resulting frequency.

4. A frequency modulation system comprising, in combination, a frequency converting device, means for generating electrical oscillations whose frequency is subject to minor variations, a rst circuit path connecting said generating means to said converting device and including means for multiplying the frequency of the oscillations generated, a modulating device connected to said circuit path for modulating the frequency of the currents therein in accordance with signals; a second circuit path connecting said generating means to said converting device and including means for multiplying the frequency of the currents generated by said generating means, said second circuit path also including means for generating electrical currents of substantially constant frequency and means for combining the currents of multiplied frequency in the second circuit path with said currents of constant frequency, and means connected to the output of said frequency converting device for transmitting the frequency modulated signal currents.

5. A frequency modulation system comprising, in combination, means for generating electrical oscillations, a first frequency converting device, a circuit connecting said converting device to said oscillation generator and comprising means for multiplying the frequency of the oscillations generated, a modulating device connected to said circuit for modulating the frequency of the currents therein, a second circuit connecting said converting device to said oscillation generator and comprising the series connection of a second frequency converting device and a device for multiplying the frequency of the oscillations generated by said oscillation generator, an oscillation generator connected to said second frequency converting device and arranged to generate current of a-frequency which is a submultiple of -the frequency to be transmitted, a frequency multiplying device connected to the output of said -rst frequency converting device and arranged to multiply the converted frequency up to that of the transmission frequency, and means for transmitting vcurrents of the transmission frequency,

.6. The process of eliminating variations in frequency due to variations in the initial source of oscillations of a lfrequency multiplying system which consists in providing a pair of paths each containing equivalent multiplying elements, providing an additional frequency, said frequency substantially eliminated, modulating the frebeing a sub-multiple of the frequency whic it is desired to transmit, combining the output Afrequency of one of said paths with said additibnal frequency, selecting the difference frequency resulting from the combination and combining said difference frequency with the output frequency of the other path to produce a third frequency which frequency is a sub-multiple of the desired transmitted frequency, and providing modulating means for affecting the frequency in the one path unequally from the effect thereof on the other, whereby the said modulations are caused to appear in the said third frequency, and transmitting a signal derived from the third frequency.

EDWIN H. ARMSTRONG. 

